Annealing is a metal heat treatment process, which means slowly heating the metal to a certain temperature, keeping it for a sufficient time, and then cooling it at an appropriate speed.
The purpose of annealing:
(1) Reduce hardness and improve machinability;
(2) Reduce residual stress, stabilize size, and reduce deformation and crack tendency;
(3) Refine grains, adjust structure, and eliminate structural defects;
(4) Make the material structure and composition uniform, improve material properties, or prepare the structure for subsequent heat treatment.
Annealing processes include: complete annealing, spheroidizing annealing, isothermal annealing, graphite annealing, diffusion annealing, stress relief annealing, incomplete annealing, post-weld annealing, etc.
The specific processes of the three commonly used annealing methods:
1. Complete annealing
Purpose: Refine grains, make the structure uniform, eliminate internal stress and processing defects, reduce hardness, and improve machinability and cold plastic deformation ability. It is used to refine the coarse overheated structure with poor mechanical properties that appear in medium and low carbon steel after casting, forging, and welding. Heat the workpiece to 30-50℃ above the temperature at which all ferrite is transformed into austenite, keep it warm for a period of time, and then slowly cool it with the furnace. During the cooling process, austenite changes again, which can make the steel structure finer.
2. Stress relief annealing is used to eliminate the internal stress of steel castings and welded parts. For steel products, the temperature below 100-200℃ at which austenite begins to form after heating, and then cools in the air after keeping warm, the internal stress can be eliminated.
3. Incomplete annealing The heating temperature is between Ac1 and Accm, and the cooling rate: above 500-600℃, carbon steel is 100-200℃/h, alloy steel is 50-100℃/h, and high alloy steel is 20-60℃/h, mainly used for hypereutectoid steel.
Annealing quality inspection standards:
Inspection scope:
1. Annealing temperature, heating time, and holding time.
2. Surface quality and hardness test of products after heat treatment annealing.
